The placement of allocation units (e.g., clusters) on a file system is sequential for purposes of access and speed of data retrieval. This is primarily due to the mechanical nature of the physical media (hard drive) access, where access times (as well as reducing physical degradation of the drive) benefit from sequential placement of allocation units across the media. Indeed, defragmentation programs exist for the purpose of keeping the allocation units of individual files sequential.
However, once the physical media is no longer secure, having sequential placement of allocation units assists in reconstructing the data, referred to as forensic analysis. For example, it is recommended that when discarded, a hard drive be wiped clean of all data or physically destroyed so that forensic analysis cannot be used to steal sensitive data. Such approaches to theft prevention are not always practiced, however, and thus forensic analysis of data remains a problem.